Exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as, but not limited to, carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system as part of an aftertreatment system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust aftertreatment technology for reducing emissions is a selective catalytic reduction (SCR) system. An SCR system converts nitrogen oxides (NOx) into diatomic nitrogen with the aid of a catalyst. Generally, SCR systems utilize a gaseous reductant typically in the form of ammonia. The ammonia is added to exhaust gases entering the SCR system and stored in the catalyst. Heat produced by the exhaust gases creates a chemical reaction between the ammonia in the catalyst and the exhaust gases causing NOx to be converted to diatomic nitrogen.
When a vehicle operates at idle, or in low exhaust temperature environments, amounts of NOx entering the SCR system may exceed amounts of NOx leaving the SCR system. In such cases, NOx may build up in the catalyst. When transitioning to normal operating temperatures, the amount of NOx leaving the SCR system may exceed the amount of NOx entering the SCR system due to NOx stored in the catalyst. During such periods, a prediction error will occur in an SCR control module which monitors SCR system efficiency. Accordingly, it is desirable to provide the SCR module with a NOx correction model to improve NOx conversion efficiency estimations.